1. Field of the Invention
This invention relates to Li-conducting lithium orthosilicates and more particularly to such orthosilicates in which a portion of the silicon is replaced by at least one of Al, P, and S and to their use as solid electrolytes in electrochemical cells.
2. Description of the Prior Art
Solid ionic conductors with Li.sup.+ as the mobile species are desirable for use as solid electrolytes.
Li has long been regarded as a desirable component of galvanic cells. It is inexpensive and its unparalleled reduction potential (E.degree..sub.red =-3.024v) and light weight have often suggested its use as the anodic component in high energy-density storage batteries. (See, for example, S. Pizzini, J. Appl. Electrochem 1, 153 (1971)). The reactivity of Li with water has required the use of non-aqueous electrolytes such as organic-soluble lithium perchlorates, or fused lithium halides. Li.sup.+ conductors which are solid would be quite useful in such utilities; these conductors also serve to separate the anodic and cathodic components.
Solid electrolytes and in particular completely solid state galvanic cells offer special advantages as low-energy density, low-power density primary batteries. These advantages include the possibility for long shelf life, broad temperature limits of operability and miniaturization. A Li.sup.+ conducting solid electrolyte would provide the basis for a lighter and lower cost alternative to the solid state batteries which rely on the transport of Ag.sup.+ in various silver halides.
One such Li.sup.+ conducting solid electrolyte is lithium orthosilicate. The structure of the compound lithium orthosilicate Li.sub.4 SiO.sub.4 was determined by H. Vollenkle, A. Wittmann and H. Nowotny, Monat. Chem. 99, 1360 (1968). It consists of isolated SiO.sub.4 tetrahedra linked by LiO.sub.n polyhedra where n=4,5,6. According to Vollenkle et al, all the lithium sites are partially occupied. Some of the SiO.sub.4 tetrahedra share edges with the LiO.sub.n polyhedra and the polyhedra are often linked together by sharing common faces.
A. R. West, J. Appl. Electrochem., 3, 327 (1973) has discussed the ionic conduction of Li.sub.4 SiO.sub.4 and related phases in which Zn.sup.2.sup.+, Mg.sup.2.sup.+, or Al.sup.3.sup.+ partially replaces Li.sup.+ and also those in which Ge.sup.4.sup.+ or Ti.sup.4.sup.+ partially replaces Si.sup.4.sup.+ . Conductivity measurements were reported for Li.sub.4 SiO.sub.4 and representative Ge-, Ti-, and Zn-containing compositions. The conductivity of Li.sub.4 SiO.sub.4 at 200.degree. C. is reported as 1 .times. 10.sup.-.sup.6 (.OMEGA.-cm).sup.-.sup.1. Ti.sup.4.sup.+ -substituted compositions resulted in conductivities higher than that of Li.sub.4 SiO.sub.4 while Zn.sup.2.+-. and Mg.sup.2.+-. substituted compositions resulted in conductivities lower than that of Li.sub.4 SiO.sub.4. The highest conductivity reported at 200.degree. C. is 5 .times. 10.sup.-.sup.5 (.OMEGA.- cm).sup.-.sup.1.
In West, there is no mention of substituting Al.sup.3.sup.+, P.sup.5.sup.+, or S.sup.6.sup.+ for some of the Si.sup.4.sup.+ with corresponding compensation in the amount of Li.sup.+ present. West does note that the change in fractional occupancy of the various lithium sites with solid solution composition cannot be determined a priori and that these lithium site occupancies will probably vary in a different manner along each solid solution series so that in order to know the occupancies (and from this to have some indication about the conductivity) for any particular composition, full refinement of the structure of a single crystal of that composition would be needed.
The known Li.sup.+ conductors with the highest conductivities are LiI mixed with CaI.sub.2 (C. R. Schlaiker and C. C. Liang, Proc. NATO Adv. Study Inst. of Fast Ion Transport, Sept. 5, 1972); single crystals of lithium .beta.-alumina LiAl.sub.11 O.sub.17 (M. S. Whittingham and R. A. Huggins, NBS Spec. Pub. 364, Solid State Chemistry, 139 (1972)) and single crystals of certain of the Cl--Br lithium haloboracites (Bither and Jeitschko, U.S. Pat. No. 3,911,085). All have conductivities at 200.degree. C. of the order of 10.sup.-.sup.3 (.OMEGA.-cm).sup.-.sup.1.
Two recently issued Japanese patents discuss the phases Li.sub.2 SO.sub.4.LiOH.LiI and Li.sub.2 SO.sub.4.LiOH with conductivities of 5 .times. 10.sup.-.sup.2 (.OMEGA.cm).sup.-.sup.1 and 1.5 .times. 10.sup.-.sup.2 (.OMEGA.cm).sup.-.sup.1, respectively at 200.degree. C. (Japanese Kokai: Nos. 49-81899 and 49-81898, issued Aug. 7, 1974). However, these phases appear to melt below 200.degree. C. and therefore are not useful as solid electrolytes at a temperature of 200.degree. C. or above.